Ok. First of all, Universal Studios, Islands of Adventure, and Seaworld are NOT all under the "universal Umbrella. That's not accurate. Universal Studios and Islands of Adventure ARE, and they are on the same property as well. Seaworld is owned by a different company, the same one who owns Bush Gardens, Discovery Cove, and the waterpark, Aquatica.
Now, Universal Studios and Islands of adventure are both very fun theme parks. My family has annual passes and we go to both parks on a regular basis. If you are very much into roller coaster and scary rides, I would plan on spending an entire day in Islands of adventure. That is where you will find most of the roller coasters and the scarier rides. They have the action hero stuff, all of the Dr. Seuss stuff, the Harry Potter stuff, most of the old cartoon stuff, and the Dinosaur stuff. Poisiodon is there too. There is a cool fountain there that "talks" and sprays water at you too. It's qute funny. There are some water rides there as well, so plan on getting wet and bring dry clothes and wear shoes that can get wet and will dry easily. (but will stay on your feet well for other rides like Harry potter and the hulk)
The Universal Studios side has rides, but they are more movie themed, and not as many are roller coaster type. They are more of a 4 d type, although some are on the roller coaster theme. They are more inside too. There are also some shows. This is the side that has the light show at night which is fairly new, the 100 years of movies show. (very cool)
You can do both parks in a day, but I wouldn't plan on it. If you are planning on riding all of the rides, or most of them, I would plan a day at each park. You can go from park to park, but it is easier to spend a whole day in one park and then go to another the next.
I would NOT go to the Harry Potter / hogworts area FIRST as you were told earlier. The lines are always long there, but that is what EVERYONE does. LOL! If you wait until lunchtime or shortly after, you will find the lines are shorter, and you will not have to wait as long. Also, if you stay on site you will have a speed pass, which gets you on the ride faster. If you can, get in the line for single rider on the Harry Potter ride. You can't really sit "together" anyway, and you pretty much ride alone on that ride anyway, and the single rider line is WAY shorter. It will save you SO much time! (on that ride you are basically sitting in a box, so you can't see who you are sitting with anyway) I would reccommend this as long as you aren't with very young kids.
If you have time while you are in Orlando, I would try to go to some of the smaller attractions. They are so much fun and not nearly as crowded as the big theme parks. Seaworld is fun, and will be a full day. Just don't forget your sunscreen. It gets really hot here! Also, you will get wet in most of the shows and rides there. (well, if you want to anyway. ) Bring a camera, and have fun! I would suggest a couple good ziploc bags for your camera, and also your money and ID's. (just in case you do get soaked) It's the way we always pack when we go! Enjoy!!